When constructing wells such as oil or gas well, it is common to drill a borehole and then line it using a steel casing. The steel casing is formed by joining a number of tubular casing sections end to end and running them into the borehole. Once the casing is in place, cement is pumped down the casing so as to exit at its lower end and return to the surface and fill the annulus between the outside of the casing and the borehole wall.
During the drilling process, boreholes sometimes take on a “corkscrew” or helical path. This most often occurs in deviated wells, and may be the result of inappropriate bottom hole assembly selection, excessive weight-on-bit, or the need for continuous trajectory corrections. As a result, when the driller tries to run casing into the borehole, problems may be encountered. The profile of the borehole may very close to a perfect circle of diameter greater than that of the casing to be run. If the casing to be run is very flexible, it will be able to follow the turns of the borehole, and all will be well. Realistically, however, casings are relatively stiff. As a result, they are often unable to comply with the borehole trajectory and may, in the limit, not be able to go downhole. In a “corkscrewed” borehole, the borehole may be locally circular, but the centre of this circle when traced along the borehole describes neither a straight line nor a smooth curve (as might be expected in a deviated well), but instead traces a helical path. This can result from the drilling process. In such a situation, a 16″ diameter borehole may be so tortuous that a 13.375″ diameter casing can become stuck due to contact with the borehole wall before it can be fully run into place. The cost of getting stuck in such situations can be very high, running into millions of dollars in extreme situations.
The problem is to determine the maximum diameter of casing that will pass through the borehole without being unduly affected by its tortuosity, irrespective of the local diameter of the borehole.
Previous proposals have been made to determine curvature and deformation of cased or lined boreholes. For example, the CALTRAN™ software product package of C-FER TECHNOLOGIES™ that uses raw caliper-log data from a multi-sensor caliper tool to determine the 3D shape of downhole tubulars. 3D drift diameter accounts for curvature and ovalisation and allows an estimate of what size tool will fit downhole.
This invention seeks to provide a method which is applicable to uncased or unlined (i.e. ‘open’) boreholes and to cased or lined wells.